An MOS transistor device such as a MOSFET (Metal Oxide Semiconductor Field-Effect Transistor) or an IGBT (Insulated Gate Bipolar Transistor) is a voltage controlled transistor device which is an on-state (conducting state) or an off-state (blocking state) dependent on a charging state of an internal capacitance. This internal capacitance is connected between a control node (gate node) and a load node (source node) and is usually referred to as gate-source capacitance. In the on-state of the transistor device, leakage currents may occur that discharge the gate-source capacitance. Excessive leakage currents may be an indication that the transistor device is defect.
One approach to evaluate the gate-source leakage current is to apply a voltage between the gate node and the source node that is higher than a drive voltage applied between the gate node and the source node during normal operation, and to measure the current flowing between the gate node and the source node. The transistor device is considered to be defect if the current is higher than a predefined threshold. Applying a high testing voltage, however causes stress that may decrease lifetime of the transistor device. It is therefore desirable to evaluate the gate-source leakage current in a softer way.